Present efforts are concentrated on evaluating the role of host reactions on parasitologic parameters (see Z0I AI 00347) and on the development of models for the kinetics of egg laying, egg destruction and egg excretion in S. mansoni and S. japonicum infections. Accurate assessment of the rate of egg destruction is important for these models. Eggs in the tissues of S. japonicum infected mice persisted in unchanged numbers in the tissues for at least 1 year after chemotherapeutic cure of infections of 8 weeks duration. The destruction of S. mansoni in murine tissues may also be slower than previously thought.